Tonight, as it occasionally happens to me, I dreamt of practicing Judo. In the dream, I was aware of having a pac implanted, but I still trained in Ne Waza, just paying attention to it. And I was being effective too, whoa! Come to think of, I realize today's date and... I wholeheartedly hope that is not to be interpreted as a destiny's April Fool's joke on me! Anyway, /me is still fighting...
Second part: as I woke up I read USENET as usual. I found an April Fool's joke in twice, because it is a recurring theme there. (Namely that Alan Moore is visiting Italy, and more precisely Florence on this particular day.)
Then, divinely inspired, I posted my own joke to isam, the/an Italian martial arts ng.
A very nice and effective joke was played to all users on Italian Wikipedia too.
I haven't noticed anything in clpmisc nor PerlMonks yet. Neither jokes played there nor barely reported. Personally, I think one of the best ones I heard of in the last few years is the Poisson d'Avril's Theorem from 2005 (now I notice it was updated in 2006) especially in view of the thread it spawned in sci.math, which I was regularly reading, back then. (Google Groups link at least for this one.)